Evaluating the Ryman–Laikre effect for marine stock enhancement and aquaculture

The Ryman–Laikre (R-L) effect is an increase in inbreeding and a reduction in total effective population size (N_eT) in a combined captive–wild system, which arises when a few captive parents produce large numbers of offspring. To facilitate evaluation of the R-L effect for scenarios that are relevant to marine stock enhancement and aquaculture, we extended the original R-L formula to explicitly account for several key factors that determine N_eT, including the numbers of captive and wild adults, the ratio of captive to wild N_e/N (β), productivity of captive and wild breeders, and removal of individuals from the wild for captive breeding. We show how to provide quantitative answers to questions such as: What scenarios lead to no loss of effective size? What is the maximum effective size that can be achieved? and What scenarios insure that N_eT will be no smaller than a specified value? Important results include the following: (1) For large marine populations, the value of β becomes increasingly important as the captive contribution increases. Captive propagation will sharply reduce N_eT unless the captive contribution is very small or β is very large (∼10³ or higher). (2) Very large values of β are only possible if wild N_e/N is tiny. Therefore, large wild populations undergoing captive enhancement at even modest levels will suffer major reductions in effective size unless wild N_e is a tiny fraction of the census size (about 10⁻⁴ or lower).

captive breeding, effective population size, inbreeding, genetic drift